Two-dimensional semiconducting transition metal dichalcogenides (“TMDCs”) have generated significant research activity and excitement in the past few years due to their promising and intriguing properties, such as layer number dependent band gaps, photoluminescence, electroluminescence, valley polarization, and catalytic activity. For other low-dimensional materials such as carbon nanotubes and graphene, chemical functionalization has been crucial for modifying their physical, electronic, optical, and chemical properties. By using chemistry to tune these properties, it is possible to engineer how these materials behave, and how they interact with their external environment for a wide range of applications including transistors and gas sensors. The chemical functionalization of TMDCs is expected to be similarly important, yet methods for doing so are in their relative infancy. Noncovalent doping of TMDCs has been demonstrated using various chemical species such as potassium, polyethyleneimine (PEI), 1,2-dichloroethane, benzyl viologen (BV), and F4TCNQ and NADH; meanwhile, covalent functionalization of the basal plane has been demonstrated using organic halides and aryl diazonium salts, as well as thiolation of chalcogen vacancies.
Covalent functionalization is beneficial for many applications because the chemical changes are more robust and stable. Previous reports of covalent functionalization have relied on first converting the semiconducting 2H phase of MoS2, WS2, and MoSe2 to the metallic 1T phase using n-butyllithium intercalation and exfoliation, (see for example the procedures described in Knirsch, K. C. et al. Basal-Plane Functionalization of Chemically Exfoliated Molybdenum Disulfide by Diazonium Salts. ACS Nano 9, 6018-6030 (2015), and in Voiry, D. et al. Covalent functionalization of monolayered transition metal dichalcogenides by phase engineering. Nature Chemistry 7, 45-49 (2015).) The lithiated forms of the TMDCs are more electron-rich, making them more amenable to some types of reactions, but they lose their semiconducting nature and photoluminescence (PL). The PL can be recovered after chemical functionalization, but at a different energy, suggesting the electronic structure is altered. Furthermore, processing using n-butyllithium is extremely hazardous because it is pyrophoric, corrosive, and flammable.